The invention is related to microphotonics, and in particular to an ultra-high-Q surface-tension-induced monolithically integrated on-chip resonator.
Optical resonators confine light for extended period and strongly enhance the interaction between light and matter. Due to their light confining nature, optical resonators have now become a focus of both scientific investigation and numerous practical applications, including lasers, optical switches, optical filters, biosensors and nonlinear optics. Quality factor Q, defined as 2 pi times the ratio of optical power dissipated in each round trip light travels in the resonator against power stored in the resonator, characterizes the ability of the resonator to confine light. Strong light-matter interaction is induced in high-Q resonators due to the enhanced optical power confinement, leading to improved device performance characteristics. For instance, in microcavity lasers, increased Q-factor suggests decrease of lasing threshold; in optical switches, high-Q helps to enhance extinction ratio and in sensor applications, improved Q enables higher sensor sensitivity.
Now highest Q-factor value is achieved in surface-tension-induced microcavities (STIMs), namely resonator structures with ultra-smooth surface formed by virtue of surface tension to reduce light scattering loss. The record 1010 Q-factor has been realized in silica microspheres made by melting the tip of a tapered silica optical fiber. However, at present, the existing STIMs are mostly off-chip, which seriously limited their device robustness in practical applications. The difficulties faced by off-chip resonators are two-fold, which lie in both STIM resonator fabrication and optical coupling scheme. On the fabrication side, the fabrication process of silica microspheres is very difficult to control and reproduce, resulting in varied resonant wavelengths for different microspheres.
The use of high temperature reflow process also makes it intrinsically inappropriate for Si-VLSI process. To resolve the issue, some on-chip resonator designs have been fabricated or proposed, nevertheless a robust coupling configuration is still to be developed. At present even the on-chip STIM resonators still require off-chip fiber evanescent coupling, which is extremely sensitive to the distance between the resonator and fiber bus and thus requires accurate alignment and often vulnerable to environmental changes.